The ordinary method of producing a titanium material is explained in the following. First, the method starts with an ingot obtained by solidifying titanium melted by the consumable electrode arc melting method or electron beam melting method, and the ingot is broken down by blooming, rolling or other hot-working process to form a slab, billet or other material for hot rolling. The material for hot rolling is hot rolled to process the slab into a flat material (plate or sheet) or the billet into a bar or rod. The hot-rolled plate, sheet, bar or rod is annealed and/or descaled into a product as is or is made into the final product by cold rolling, cold drawing or other cold-working process and annealing. Note that although surface defects are removed by the descaling after hot rolling, the surface must be removed deeper in proportion as the surface defects are deeper, so that yield naturally declines.
On the other hand, in the electron beam melting method or plasma arc melting method in which melting is done at a location apart from the mold and the molten titanium is poured into the mold, the freedom of mold shape is high, which makes use of a rectangular or cylindrical mold possible. In the case of producing flat material from a rectangular ingot, or in the case of producing bar or rod from a cylindrical ingot, with consideration to the point of the ingot shape, it becomes possible to omit the aforesaid breakdown process, thus lowering production cost.
However, the solidified structure of an industrially utilized large ingot is composed of coarse crystal grains of up to several tens of mm, and when directly hot rolled without passing through a breakdown process experiences uneven deformation owing to the coarse crystal grains, with growth of large surface defects sometimes occurring. As a result, yield declines considerably during, for example, the descaling for removal of surface defects after hot rolling, and product inspection.
Further, when the flat material or strip coil is hot rolled, large wrinkles caused by the coarse solidified structure occur not only on the sheet surface but also at the side surfaces and corners, and these wrinkles wrap around to the sheet surface side to become surface defects called seam defects and develop into edge cracks and the like.
Also during rolling of bar or rod, surface defects occur owing to the formation of wrinkles on the free surface portions and the flash not in contact with the rolls, just as on the side surfaces of a flat material of strip coil during hot rolling. In the aforesaid ordinary production method, the ingot is broken down under heating and formed into a slab or billet of a size that can be hot rolled. However, depending on the amount of hot working and/or the working method during the breakdown, the amount of deformation of the portion constrained by the frictional resistance at the contact region with the working tool is small, so that a so-called dead metal zone occurs. Even if breakdown is conducted, the deformation of this dead metal zone is small and the coarse solidified structure of the ingot remains, so that, similarly to the above, surface defects like those mentioned above sometimes occur when the flat material, bar or rod is thereafter hot rolled.
A need is therefore felt for a titanium material for hot rolling by which the coarse solidified structure of the ingot, or the remainder thereof, does not develop into harmful surface defects in the ensuing hot rolling process.
Patent Document 1 proposes a method wherein, in the case of directly hot-working an ingot of titanium material, strain is imparted to the surface layer to refine the crystal grains near the surface, the surface is then recrystallized to a depth of 2 mm or greater by heating to the recrystallization temperature or higher, and hot working is then conducted. As the means for imparting strain can be mentioned forging (pressing), roll reduction, shot blasting and the like.
Although Patent Document 1 cites shot blasting as the means for imparting strain, the depth of strain formed by ordinary shot blasting is on the order of 300 to 500 μm or less, which is very small relative to the coarse solidified structure of several tens of mm, and, as explained later, the surface defects are by no means suppressed.
In order to form a deep recrystallization layer, it is substantially necessary in the method set out in Patent Document 1 to impart strain to a deep level by forging or roll reduction. However, although forging or roll reduction using ordinary tools forms a deep recrystallization layer, cases occur in which, as explained later, surface defects are not suppressed but, to the contrary, the incidence of surface defects increases.